a. Chemiluminescent Detection of Phosphatase Enzymes
Hydrolytic enzymes such as alkaline phosphatase are frequently used as markers or labels in enzyme-linked assays for biological molecules and other analytes of interest such as drugs, hormones, steroids and cancer markers. In addition, phosphatase enzymes, e.g. alkaline phosphatase (AP) and acid phosphatase (AcP), are clinically significant in their own right in human and veterinary diagnostics. Chemiluminescent detection of these enzymes offers a safe, convenient and sensitive means to provide a quantitative measure of the amount of enzyme in a sample or of the amount of an enzyme-labeled analyte or labeled specific binding partner for an analyte. Numerous chemiluminescent reaction schemes have been developed to quantitate the level of particular hydrolytic enzymes. Most of these schemes are complex and expensive, requiring multiple enzymes or several reagents. Commercial acceptance of most of such methods for large volume testing has been slow.
Applicant's co-pending U.S. patent application Ser. No. 08/585,090 which is fully incorporated herein by reference, discloses the chemiluminescent reaction of certain heterocyclic compounds bearing an enol phosphate group with a phosphatase enzyme. Light emission is enhanced in the presence of cationic surfactants allowing the phosphatase to be detected at levels of 10.sup.-18 to 10.sup.-19 mol.
Applicant's co-pending U.S. patent application Ser. No. 08/683,927 which is fully incorporated herein by reference, discloses the use of cationic aromatic compounds (CAC's) in conjunction with the chemiluminescent reaction of certain heterocyclic compounds bearing an enol phosphate group with a phosphatase enzyme to substantially increase the amount of light emitted. The detection limit of phosphatase enzymes is thereby dramatically lowered.
b. Chemically and Enzymatically Triggerable Dioxetanes
Stable 1,2-dioxetanes bearing a protected phenol group triggering group undergo a chemiluminescent decomposition upon removal of a protecting group (A. P. Schaap, T. S. Chen, R. S. Handley, R. DeSilva, and B. P. Giri, Tetrahedron Lett., 1155 (1987); A. P. Schaap, R. S. Handley, and B. P. Giri, Tetrahedron Lett., 935 (1987); A. P. Schaap, M. D. Sandison, and R. S. Handley, Tetrahedron Lett., 1159 (1987); and A. P. Schaap, Photochem. Photobiol., 47S, 50S (1988)). Enzymatically triggerable dioxetanes bear an aryloxide substituent which is blocked by an enzymatically removable protecting group. Reaction with a hydrolytic enzyme in an aqueous buffer reveals an aryloxide anion which accelerates the chemiluminescent decomposition rate of the dioxetane by orders of magnitude. Chemically triggerable dioxetanes bear an aryloxide substituent which is blocked by a protecting group which is removed by a simple chemical agent. An example is deprotection of an acetoxy dioxetane with hydroxide or a silyloxy dioxetane with fluoride. Numerous examples of such triggerable dioxetanes are disclosed, for example, in U.S. Pat. Nos. 4,857,652, 5,068,339, 4,952,707, 5,112,960, 5,220,005, 5,326,882 and in PCT applications WO96/24849, WO94/10258 and WO94/26726. However, an inherent disadvantage of some triggerable dioxetanes is their tendency to generate background chemiluminescence in the absence of enzyme through slow thermal decomposition or non-enzymatic hydrolysis.
c. Luminol Derivatives
A phosphate and a NAG derivative of luminol are known (K. Sasamoto, Y. Ohkura, Chem. Pharm. Bull., 38, 1323-5 (1991); M. Nakazono, H. Nohta, K. Sasamoto, Y. Ohkura, Anal. Sci., 8, 779-83 (1992)). Treatment of the luminol derivative with the appropriate enzyme liberates luminol which is reacted in a subsequent step with ferricyanide to produce light.
d. Luciferin Derivatives
Phosphate and galactoside derivatives of firefly luciferin are known (N. Ugarova, Y. Vosny, G. Kutuzova, I. Dementieva, Biolum. and Chemilum. New Perspectives, P. Stanley and L. J. Kricka, eds., Wiley, Chichester, 511-4 (1981); W. Miska, R. Geiger, J. Biolumin. Chemilumin., 4, 119-28 (1989)). Treatment of the firefly luciferin derivative with the appropriate enzyme liberates firefly luciferin which is reacted in a second step with luciferase and ATP to produce light.
e. Reactions Involving the Generation of Reducing Agents
Chemiluminescent methods involving the generation of a reducing agent from a phosphate ester catalyzed by alkaline phosphatase have been reported. (M. Maeda, A. Tsuji, K. H. Yang, S. Kamada, Biolum. and Chemilum. Current Status, 119-22 (1991); M. Kitamura, M. Maeda, A. Tsuji, J. Biolumin. Chemilumin., 10, 1-7 (1995); H. Sasamoto, M. Maeda, A. Tsuji, Anal. Chim. Acta, 306, 161-6 (1995)).The reducing agent causes a reaction between oxygen and lucigenin to produce light arising from the lucigenin. Representative reducing agents include ascorbic acid, glycerol, NADH, dihydroxyacetone, cortisol and phenacyl alcohol. These methods are distinguished from the present invention which involves the production of light from the deprotected fluorescent compound, not from lucigenin. The known methods of enzymatically generating a reducing agent for reaction with lucigenin all require a separate preliminary incubation step between the enzyme and the phosphate compound. This adds additional complexity and assay time.
U.S. Pat. No. 5,589,328 to Mahant discloses a chemiluminescent reaction whereby indoxyl esters, thioindoxyl esters and benzofuran esters are hydrolyzed by an enzyme and thereby generate superoxide. Luminescence is amplified by adding a chemiluminescence generating reagent such as lucigenin. Lucigenin produces chemiluminescence by reaction with superoxide.
f. Coupled Enzyme Methods
Numerous other chemiluminescent methods and assays for determining hydrolytic enzymes such as phosphatase enzymes through coupled enzyme reactions are known. A compilation of such methods is listed in A. Tsuji, M. Maeda, H. Arakawa, Anal. Sci., 5, 497-506 (1989). Other examples of dual enzyme chemiluminescent reactions are described in U.S. Pat. No. 5,306,621 and commonly assigned application Ser. No. 08/300,367. The former describes the enzymatic generation of a peroxidase enhancer to enhance the chemiluminescent oxidation of luminol with a peroxidase; the latter describes the enzymatic generation of a peroxidase enhancer to enhance the chemiluminescent oxidation of an acridancarboxylic acid derivative with a peroxidase.
With the exception of enzyme-triggered dioxetanes, each of the aforementioned methods suffers the drawback of requiring multiple reagents or enzymes in order to generate the luminescent signal. The added expense or operational complexity has hindered commercial acceptance of these methods in spite of their demonstrated exceptional detection sensitivity. Chemiluminescent methods for detecting and quantitating hydrolytic enzymes which achieve these levels of sensitivity but do not require additional enzymes or auxiliary reagents in addition to the enzyme substrate would be advantageous. The present invention provides such methods and compounds.